Switching and display devices containing ferroelectric liquid crystal mixtures ("FLC light valves") are known for example, from EP-B 0,032,362 (=U.S. Pat. No. 4,367,924). Liquid crystal light valves are devices which change their optical transmission properties, for example as a result of electrical switching, in such a manner that transmitted (and in some cases, again reflected) light is intensity-modulated. The known wrist watch and pocket calculator displays or liquid crystal displays in the areas of OA (office automation) or TV (television) are examples. These also include optical shutters, so-called "light shutters", such as are used, for example, in copying machines, printers, welding goggles, polaroid spectacles for three-dimensional viewing and the like. The application range of liquid crystal light valves also includes so-called "spatial light modulators" (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054 and articles quoted therein).
The structure of the electrooptical switching and display devices is such that the FLC layer is enclosed on both sides by layers which usually contain, in the following order starting from the FLC layer, at least one orientation layer, electrodes and an outer sheet (e.g. made of glass). Moreover, they contain a polarizer, if they are operated in the "guest-host" or in the reflective mode, or two polarizers, if the transmissive birefringence mode is utilized. If desired, the switching and display elements can contain further auxiliary layers, such as, for example, diffusion barrier or insulating layers.
These types of orientation layers together with a sufficiently small spacing of the outer sheets bring the FLC molecules of the FLC mixture into a configuration at which the longitudinal axes of the molecules are parallel to one another and the smectic planes are arranged perpendicular or inclined to the orientation layer. In this arrangement, as is known, the molecules have two equivalent orientations between which they can be switched back and forth by applying a pulsed electric field, i.e. FLC displays are capable of bistable switching. The switching times are inversely proportional to the spontaneous polarization of the FLC mixture and are in the range of .mu.s.
The main advantage of the FLC displays compared with the LC displays which up till now are still found in industrial practice for the most part is considered to be the attainable multiplex ratio, i.e. the maximum number of lines which can be addressed in a time-sequential process ("multiplex process"), which, in comparison with conventional LC displays is virtually unlimited in FLC displays. This electrical addressing is essentially based on the abovementioned pulse addressing described in SID 85 DIGEST p. 131 (1985) by way of example.
A known problem of FLC displays is the sensitivity of the orientation of the liquid-crystalline material in the liquid crystal layer to deformations caused by mechanical or thermal stress (see, for example, S. T. Lagerwall et al., Ferroelectrics 94, 3-65 (1989 )--most often described by the term shock instability.
The root of this problem, which does not occur with nematic displays, resides in an intrinsic property of ferroelectric liquid crystals--the existence of the smectic layer structure. The high flow viscosity prevents spontaneous healing of defects of the appropriate layer structure caused by outside forces. In general, the original geometry can only be regenerated by heating the display to form the nematic or isotropic phase, which requires comparatively high temperatures and is technically complicated.
Since the shock instability is caused by the intrinsic properties of smectic liquid crystals described above, this has hitherto been considered a particularly serious problem.
Previous attempts at solving the shock problem have been based on avoiding possible deformations by constructing a more stable display structure (e.g. use of structured photoresist spacers (Leti, France))--or suspending the display in an elastic frame (CANON, Japan). The disadvantages of these displays are the higher manufacturing costs for such a display and the observation that strong forces nevertheless lead to a destruction of the smectic layers.
Surprisingly, it has now been found that by using switching and display devices containing an FLC mixture which contains a coronand or cryptand it is possible to bring the FLC material into a homogeneous "bookshelf" or "quasi bookshelf" geometry (see for these geometries Dubal et al., Proc. 6th Intl. Symp. on Electrets, Oxford, England 1988; Y. Sato et al., Jap. J. Appl. Phys. 28, L 483 (1989)). This structure is more resistant to the effect of deformation than the so-called "chevron geometry" which has tilted smectic layers.